Three patients with histologically differing lesions of synovial origin and two with synovial cysts, one of which was a dissecting popliteal cyst, were examined by magnetic resonance imaging (MR) and computerized tomography (CT). The three histologically proven synovial lesions were synovial sarcoma, diffuse giant cell tumor of tendon sheath, and synovial chondromatosis. In two of the five patients MR provided better anatomic and morphologic appreciation than CT, while in the others they were of equal value. CT demonstrated calcification in two of the lesions while on MR calcification could be identified in only one patient where it outlined the mass. MR did not demonstrate calcification in the substance of the diffuse giant cell tumor of tendon sheath. Coronal, transverse, and sagittal images of magnetic resonance graphically demonstrated the extent of the soft tissue masses and their relationship to bone, vessels, and soft tissue structures. Synovial sarcoma had a shorter T1 than diffuse giant cell tumor of tendon sheath (these two lesions being of comparable size) and also had a uniformly longer T2. The dissecting popliteal cyst showed the most intense signals on the T1 weighted images, while the uncomplicated synovial cyst showed a long T1. On the T2 weighted images, each type of cyst showed a long T2. The variance and overlap of intensity of MR signals suggest limited specificity in predicting the histologic nature of the synovial lesion.